CN114657475A - Liquid lead-bismuth corrosion resistant austenitic stainless steel for high-temperature fastener and preparation method thereof - Google Patents

Liquid lead-bismuth corrosion resistant austenitic stainless steel for high-temperature fastener and preparation method thereof Download PDF

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CN114657475A
CN114657475A CN202210209665.8A CN202210209665A CN114657475A CN 114657475 A CN114657475 A CN 114657475A CN 202210209665 A CN202210209665 A CN 202210209665A CN 114657475 A CN114657475 A CN 114657475A
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stainless steel
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CN114657475B (en
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严伟
徐海涛
戎利建
刘新鹏
史显波
韩丽青
张舒展
燕春光
杨红义
李依依
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Institute of Metal Research of CAS
China Institute of Atomic of Energy
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China Institute of Atomic of Energy
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    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C33/00—Making ferrous alloys
    • C22C33/04—Making ferrous alloys by melting
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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Abstract

The invention belongs to the field of high-temperature austenitic stainless steel structural materials, and particularly relates to liquid lead bismuth corrosion resistant austenitic stainless steel for a high-temperature fastener and a preparation method thereof. The steel comprises the following chemical components in percentage by weight: c: 0.06-0.12%; si: 2.0-3.0%; mn: 0 to 1.0 percent; s: 0 to 0.005%; p: 0 to 0.01 percent; cr: 13.0 to 17.0 percent; ni: 8.0-15.0%; cu: 0 to 1.0 percent; mo: 0.5-2.0%; nb: 8 is multiplied by 100C to 1.0 percent; o: 0 to 0.003%; n: 0 to 0.03 percent; the balance being Fe. According to the invention, through component design and tissue regulation, the austenitic stainless steel for the fastener, which has high heat strength, high endurance resistance, excellent lead-bismuth corrosion resistance and excellent stress relaxation resistance, is obtained, and the stainless steel can be used as a novel fastener structure material in the nuclear energy field facing a high-temperature lead-bismuth corrosion environment.

Description

Liquid lead-bismuth corrosion resistant austenitic stainless steel for high-temperature fastener and preparation method thereof
Technical Field
The invention belongs to the field of high-temperature austenitic stainless steel structural materials, and particularly relates to liquid lead bismuth corrosion resistant austenitic stainless steel for a high-temperature fastener and a preparation method thereof.
Background
The lead-cooled fast reactor is a fast neutron reactor cooled by adopting liquid lead or lead-bismuth alloy. As one of six main reactor types of a fourth generation reactor, the lead-cooled fast reactor can well meet the target requirements of safety, economy, persistence and nuclear non-diffusion, is concerned about internationally and has wide development space in the future. Because the environment in the lead-cooled fast reactor is severe, the structural material is subjected to strong corrosion of liquid metal in the service process except for bearing high temperature, stress and irradiation environments, and the conventional material cannot be directly used to meet the long-term service requirement. Therefore, the selection of different component materials is the key to limit the development and application of the lead-cooled fast reactor technology.
The fastener is one of the structural components in the lead-cooled fast reactor, plays a very key role in connection and is concerned with the safe operation of the components in the reactor. In view of the severe service environment in the lead-cooled fast reactor, the fastener material is required to have higher high-temperature strength and good endurance resistance, more importantly, the fastener material also has excellent liquid lead-bismuth corrosion resistance and stress relaxation resistance, and the difficulty is that no steel grade for reference and reference exists at home and abroad. Therefore, how to obtain a fastener material which simultaneously has high heat strength, high endurance resistance, lead-bismuth corrosion resistance and excellent stress relaxation resistance through component design and tissue regulation is one of the key problems which must be solved in the development and application of the lead-cold fast reactor technology.
Disclosure of Invention
The invention aims to provide liquid lead-bismuth corrosion resistant and stress relaxation resistant austenitic stainless steel for a high-temperature fastener and a preparation method thereof, and the austenitic stainless steel material for the fastener, which simultaneously has high heat strength, high endurance resistance, excellent lead-bismuth corrosion resistance and excellent stress relaxation resistance, is obtained.
The technical scheme of the invention is as follows:
the liquid lead-bismuth corrosion resistant austenitic stainless steel for the high-temperature fastener comprises the following chemical components in percentage by weight:
c: 0.06-0.12%; si: 2.0-3.0%; mn: 0 to 1.0 percent; s: 0 to 0.005%; p: 0 to 0.01 percent; cr: 13.0 to 17.0 percent; ni: 8.0-15.0%; cu: 0 to 1.0 percent; mo: 0.5-2.0%; nb: 8 × 100C-1.0%; o: 0 to 0.003%; n: 0 to 0.03 percent; the balance being Fe.
The liquid lead-bismuth corrosion resistant austenitic stainless steel for the high-temperature fastener is characterized in that the chromium equivalent is calculated according to the formula (1):
cr equivalent (100X) (Cr + Mo +1.5Si +0.5Nb) (1)
The nickel equivalent is calculated according to formula (2):
ni equivalent of 100 × (Ni +30 × C +0.5 × Mn +0.5 × Cu) (2)
The Cr equivalent and the Ni equivalent satisfy: cr equivalent < 20; ni equivalent > 14.
The high-temperature fastener is made of liquid lead bismuth corrosion resistant austenitic stainless steel, and the structure of the stainless steel is single austenite.
The preparation method of the liquid lead-bismuth corrosion resistant austenitic stainless steel for the high-temperature fastener comprises the following steps:
(1) mixing raw materials according to the requirements of chemical components, and obtaining a steel ingot through vacuum induction smelting and vacuum consumable smelting;
(2) homogenizing a steel ingot, wherein the homogenizing process comprises the following steps: the charging temperature of the steel ingot is less than 700 ℃, and the temperature is raised to 1100-1200 ℃ along with the charging temperature and is kept for more than 4 hours;
(3) forging the homogenized steel ingot at the initial forging temperature of 1080-1180 ℃ and the final forging temperature of 850-950 ℃, repeatedly forging the steel ingot at a large reduction in the longitudinal-transverse-longitudinal directions for not less than 3 times, wherein the single deformation is more than 10%, the total forging ratio is more than 20, and air-cooling the steel ingot to room temperature after forging;
(4) cold deformation with deformation of more than 30% is carried out after forging;
(5) and carrying out heat treatment after cold deformation.
Preferably, in the step (3), the single deformation is greater than 12%, and the total forging ratio is greater than 25; in step (4), the deformation is > 35%.
The preparation method of the liquid lead bismuth corrosion resistant austenitic stainless steel for the high-temperature fastener comprises the following heat treatment process in the step (5):
(1) preserving the heat for 0.5 to 2 hours at the temperature of 1000 to 1150 ℃, and cooling the mixture to room temperature in air;
(2) keeping the temperature at 800-900 ℃ for 2-4 hours, and cooling in air to room temperature.
According to the preparation method of the liquid lead-bismuth corrosion resistant austenitic stainless steel for the high-temperature fastener, the yield strength of the stainless steel at 550 ℃ is more than or equal to 200MPa, the tensile strength is more than or equal to 480MPa, and the elongation is more than or equal to 40.0%.
According to the preparation method of the liquid lead bismuth corrosion resistant austenitic stainless steel for the high-temperature fastener, the thickness of an oxide film of the stainless steel is not more than 30 micrometers after the stainless steel is corroded in a liquid lead bismuth alloy (45% Pb-Bi) with saturated oxygen concentration and 550 ℃ for 3000 hours, and the stainless steel has excellent liquid lead bismuth corrosion resistance.
According to the preparation method of the liquid lead-bismuth corrosion resistant austenitic stainless steel for the high-temperature fastener, the lasting fracture time of the stainless steel is more than 1000 hours at 550 ℃ and 260MPa stress.
According to the preparation method of the liquid lead-bismuth corrosion resistant austenitic stainless steel for the high-temperature fastener, the initial stress of the stainless steel is 120MPa at 550 ℃, and the residual stress is more than 70MPa after the stainless steel is kept for 400 hours.
The design concept of the invention is as follows:
according to the invention, a proper amount of Si element is added into the austenitic stainless steel, so that the oxidation resistance of the Cr element in the austenitic stainless steel is enhanced, a compact Cr-rich and Si-rich oxide layer is generated under a high-temperature condition, and the excellent liquid lead-bismuth corrosion resistance of the steel is ensured;
according to the invention, the Mo element is added to improve the heat strength of the steel, so that the high initial high-temperature strength and long-term good endurance resistance of the steel are ensured;
according to the invention, C, Nb and Cu are added to form a high-density nano-sized NbC and Cu-rich precipitated phase, so that excellent stress relaxation resistance and creep resistance of the steel are ensured;
according to the invention, the Cr equivalent and the Ni equivalent are regulated and controlled through the proportion of different elements, so that a single austenite structure is obtained, and the high-temperature long-term structure stability of the steel is ensured.
The above component design is the prerequisite basis that the material has good comprehensive properties, and the tissue regulation and control is the guarantee that each performance is further improved. The method adopts the measures of vacuum induction and vacuum self-consumption double-vacuum purification smelting, and control of heat, cold processing, heat treatment and the like to regulate and control the structure parameters such as purity, grain size, carbide number density and dislocation density, thereby ensuring the further improvement of various properties of the steel.
The contents of main elements in the present invention are explained as follows:
C：0.06～0.12wt％
c is one of the most effective elements for increasing the Ni equivalent, and can enlarge the austenite phase region and stabilize the austenite structure. The other important role of C in the steel is to form nano-sized NbC with Nb, form high-density fine dispersed NbC particle pinning dislocation in the structure and improve the high-temperature stress relaxation resistance and creep strength of the steel. The content of C and the content of Nb in the steel follow the principle of ideal chemical proportion, and the content of Nb is ensured to be 8 times of the content of C. If the content of C or Nb is too low, the formed NbC has low density and small effect; too high C content may form M with Cr element in steel at an early stage23C6Carbide, on the contrary, deteriorates the overall performance. Therefore, the C content of the steel is 0.06-0.12 wt%.
Si：2.0～3.0wt％
The bonding force of Si and O is strong. Therefore, the thermal stability of the Si oxide is extremely strong. Si first combines with O in the ambient in an oxygen-containing ambient to form an oxide of Si. Si is added into the steel, and the Si is preferentially oxidized under a high-temperature environment to form an oxide barrier containing Si, so that further corrosion of the external environment can be prevented. By utilizing the function of Si, a proper amount of Si is added into the steel to play a role in excellent liquid lead bismuth corrosion resistance. The solid solubility of Si in austenitic steel is larger, more than 2.0 wt% of Si can be added to form a continuous compact corrosion-resistant 'barrier' with stronger binding force, but Si is a stronger ferrite forming element, and excessive Si can embrittle the steel. Therefore, the Si content in the steel is 2.0-3.0 wt% in comprehensive consideration.
Cr：13.0～17.0wt％
Cr is one of the basic elements in austenitic stainless steels. The stainless and corrosion resistance of austenitic stainless steels is obtained mainly as a result of the Cr promoting the passivation of the steel and keeping the steel in a stable passive state. Also, this action of Cr causes continuous dense Cr to be formed on the surface of the steel2O3The passive film can block ion migration and element dissolution to liquid lead and bismuth, so that the liquid metal corrosion resistance of the steel is improved, the effect of Cr and Si are mutually enhanced, and the liquid lead and bismuth corrosion resistance is better. However, Cr is a ferrite-forming element and easily forms M with C23C6. Therefore, the content of Cr is controlled to 13.0 to 17.0 wt%.
Ni：8.0～15.0wt％
Ni is another basic element in austenitic stainless steels. The main function is to form and stabilize austenite, so that the steel obtains a completely austenitic structure, and the thermodynamic stability of austenitic stainless steel is improved. However, the solubility of Ni in the liquid lead-bismuth alloy is relatively high, and the liquid metal corrosion resistance is deteriorated due to excessively high content of Ni; at the same time, an increase in the Ni content in the steel decreases the solubility of C in austenitic stainless steel, thereby increasing the carbide precipitation tendency. Therefore, the Ni content in the steel is controlled to be 8.0-15.0 wt% in comprehensive consideration.
Cu：0～1.0wt％
Cu is a non-carbide forming element, a nano-sized Cu-rich phase can be separated out in the heat treatment and long-term service process when Cu is added into the austenitic steel, the coarsening rate of the nano-sized Cu-rich phase is low, and the nano-sized Cu-rich phase can play a role in pinning dislocation so as to improve the stress relaxation resistance and the endurance strength. Meanwhile, the addition of Cu into the austenitic stainless steel can obviously reduce the cold hardening tendency of the chromium-nickel austenitic stainless steel and improve the cold working forming performance. However, excessive Cu deteriorates the hot workability of the material. Therefore, the content of the Cu element added into the steel is 0-1.0 wt%.
Mo：0.5～2.0wt％
Mo is an element that forms and stabilizes and expands the ferrite phase region, and in order to maintain a single austenite structure, the content of Ni is increased while adding Mo to the steel of the present invention. The main role of Mo in the steel of the present invention is to improve the high temperature strength of the steel. The high temperature durability of the steel increases as the Mo content in the steel increases, but Mo promotes intermetallic phases in austenitic stainless steels, such as: the separation of sigma phase and Laves phase reduces the stability of the structure. Therefore, the Mo content in the steel of the present invention is 0.5 to 2.0 wt% in comprehensive consideration.
Nb：8×100C～1.0wt％
Nb is a key element in the steel and is the basis for ensuring excellent stress relaxation resistance. Nb and C in the steel form a high-density NbC nano-sized precipitated phase, and the elastic strain is prevented from changing into plastic through pinning dislocation of the precipitated phaseThe strain shifts and higher residual stresses are maintained. According to a rough calculation, the Nb content required to fix all C in austenite to NbC was 7.78 times the C content. The minimum Nb content in the steel according to the invention is 8 times the C content, considering that Nb is also partially consumed by forming nitrides corresponding to the trace amounts of N in the steel. Since Nb is an easily segregating element, and excessive Nb in the steel forms Fe after long-term aging2Nb type Laves phase, deteriorating performance. Thus, taken together, the maximum amount of Nb is no more than 1.0 wt.%.
The invention has the advantages and beneficial effects that:
1. according to the invention, by adding a proper amount of Si element, the antioxidation of Cr element in the austenitic stainless steel is enhanced, continuous and compact Cr-rich and Si-rich oxide layers are generated under a high-temperature condition, and the liquid lead-bismuth corrosion resistance is greatly improved;
2. c, Nb and Cu are added into the steel, and a hot working, cold working and hot treatment tissue regulation and control means are utilized to obtain high-density nanometer-sized NbC and Cu-rich precipitated phase, so that excellent stress relaxation resistance is obtained;
3. the steel of the invention obtains a single austenite structure by controlling and balancing the Cr equivalent and the Ni equivalent, thereby ensuring good comprehensive performance;
4. the steel can be applied to a novel fastener structure material facing a high-temperature lead-bismuth corrosion environment in the field of nuclear energy.
Drawings
FIG. 1 is the microstructure of example 1.
FIG. 2 is the microstructure of example 2.
FIG. 3 is a graph of residual stress versus time at 550 ℃ for the steels of example 5 and comparative example 1, with an initial stress of 120 MPa.
FIG. 4 shows the oxide film morphology of comparative example 1 after 3000 hours of corrosion in a liquid lead bismuth alloy (45% Pb-Bi) at 550 ℃ under saturated oxygen concentration.
FIG. 5 shows the oxide film morphology of example 4 after etching in a liquid lead-bismuth alloy (45% Pb-Bi) at 550 ℃ for 3000 hours at a saturated oxygen concentration.
FIG. 6 shows the oxide film morphology of example 5 after etching in a liquid lead-bismuth alloy (45% Pb-Bi) at 550 ℃ for 3000 hours at a saturated oxygen concentration.
Detailed Description
In the specific implementation, the preparation method of the steel (example) of the invention is as follows:
(1) mixing raw materials according to the chemical components, and carrying out vacuum induction smelting and pouring to obtain a steel ingot;
(2) removing surface oxide skin of the steel ingot obtained by vacuum induction smelting, and cutting two ends to prepare a consumable electrode bar;
(3) further purifying and smelting the consumable electrode bar in a vacuum consumable smelting furnace to obtain a high-purity consumable steel ingot;
(4) the steel ingot is subjected to heat preservation at 1150 ℃, is subjected to heat preservation for 8 hours and then is forged, the initial forging temperature is 1110 ℃, the initial forging is subjected to repeated large-reduction forging in the longitudinal direction, the transverse direction and the longitudinal direction, the repeated times are 3 times, the deformation of each forging is about 15 percent, and the total forging ratio is about 28; then forging the round bars into round bars with different diameters, wherein the finish forging temperature is 900 ℃, and air cooling the round bars to room temperature after forging;
(5) cold drawing the forged round bar at room temperature to obtain a deformation of about 40%;
(6) cutting a related performance sample of the bar stock after cold deformation, firstly preserving heat for 1 hour at 1050 ℃, and cooling in air to room temperature; then, the temperature is kept at 850 ℃ for 3 hours, and the air cooling is carried out to the room temperature.
The comparative example 1 steel was a commercial type 316 austenitic stainless steel.
Hereinafter, the present invention will be described by comparing various examples and comparative examples, which are for illustrative purposes only and the present invention is not limited to these examples.
Example 1
The steel comprises the following chemical components in percentage by weight: c: 0.065%; si: 2.17 percent; mn: 0.43 percent; s: 0.0017%; p: 0.009%; cr: 14.55 percent; ni: 9.5 percent; mo: 0.05 percent; nb: 0.68 percent; o: 0.002%; n: 0.005 percent; the balance being Fe. Wherein the Cr equivalent is 18.20<20, and the Ni equivalent is 11.96 no more than 14.
Example 2
The steel comprises the following chemical components in percentage by weight: c: 0.081%; si: 2.58 percent; mn: 0.60 percent; s: 0.0016 percent; p: 0.008 percent; cr: 14.70 percent; ni: 12.75 percent; cu: 0.64 of; mo: 0.56 percent; nb: 0.80 percent; o: 0.002%; n: 0.005 percent; the balance being Fe. Wherein the Cr equivalent is 19.53<20 and the Ni equivalent is 15.80> 14.
Example 3
The steel comprises the following chemical components in percentage by weight: c: 0.09%; si: 2.47 percent; mn: 0.58 percent; s: 0.0017%; p: 0.008 percent; cr: 14.60 percent; ni: 14.75 percent; mo: 1.06 percent; nb: 0.85 percent; o: 0.002%; n: 0.005 percent; the balance being Fe. Wherein the Cr equivalent is 19.79<20 and the Ni equivalent is 17.74> 14.
Example 4
The steel comprises the following chemical components in percentage by weight: c: 0.12 percent; si: 2.57 percent; mn: 0.51 percent; s: 0.0015 percent; p: 0.008 percent; cr: 14.73%; ni: 10.26 percent; mo: 0.03 percent; nb: 0.96 percent; o: 0.0016 percent; n: 0.004 percent; the balance being Fe. Wherein the Cr equivalent is 19.09<20, and the Ni equivalent is 14.12> 14.
Example 5
The steel comprises the following chemical components in percentage by weight: c: 0.11 percent; si: 2.3 percent; mn: 0.81 percent; s: 0.0015 percent; p: 0.008 percent; cr: 14.2 percent; ni: 12.24 percent; cu: 0.90; mo: 1.53 percent; nb: 0.90 percent; o: 0.0016 percent; n: 0.004%; the balance being Fe. Wherein the Cr equivalent is 19.63<20 and the Ni equivalent is 16.39> 14.
Example 6
The steel comprises the following chemical components in percentage by weight: c: 0.11 percent; si: 2.44 percent; mn: 0.61%; s: 0.0014%; p: 0.007%; cr: 14.3 percent; ni: 11.22 percent; mo: 1.03 percent; nb: 0.42 percent; o: 0.0017%; n: 0.004 percent; the balance being Fe. Wherein the Cr equivalent is 19.2<20 and the Ni equivalent is 14.82> 14.
Comparative example 1
The steel comprises the following chemical components in percentage by weight: c: 0.022%; si: 0.34 percent; mn: 1.36 percent; s: 0.023%; p: 0.03 percent; cr: 18.24 percent; ni: 12.36 percent; mo: 2.53 percent; n: 0.12 percent; the balance being Fe.
Comparative example 2
The steel comprises the following chemical components in percentage by weight: c: 0.11 percent; si: 2.3 percent; mn: 0.81 percent; s: 0.0015 percent; p: 0.008 percent; cr: 14.2 percent; ni: 12.24 percent; cu: 0.90; mo: 1.53 percent; nb: 0.90 percent; o: 0.0016 percent; n: 0.004%; the balance being Fe. Wherein the Cr equivalent is 19.63<20 and the Ni equivalent is 15.94> 14.
Comparative example 2 differs from example 5 in that comparative example 2 is not cold-deformed and the other manufacturing processes are the same.
Comparative example 3
The steel comprises the following chemical components in percentage by weight: c: 0.11 percent; si: 2.3 percent; mn: 0.81 percent; s: 0.0015 percent; p: 0.008 percent; cr: 14.2 percent; ni: 12.24 percent; cu: 0.90; mo: 1.53 percent; nb: 0.90 percent; o: 0.0016 percent; n: 0.004%; the balance being Fe. Wherein the Cr equivalent is 19.63<20 and the Ni equivalent is 15.94> 14.
Comparative example 3 unlike example 5, the conventional hot forging process of comparative example 3 was: and (3) keeping the temperature of the steel ingot at 1150 ℃, forging the steel ingot after keeping the temperature for 8 hours, wherein the initial forging temperature is 1110 ℃, the final forging temperature is 900 ℃, forging the steel ingot into a round bar, the total forging ratio is about 8, air cooling the steel ingot to room temperature after forging, and other preparation processes are the same.
The 550 ℃ high temperature strength of the above examples and comparative examples is shown in Table 1.
TABLE 1
Numbering Yield strength (MPa) Tensile strength (MPa) Elongation (%)
Example 1 136 387 31
Example 2 209 492 41
Example 3 216 508 53
Example 4 168 475 45
Example 5 221 512 47
Example 6 203 487 44
Comparative example 1 159 467 58
Comparative example 2 205 491 50
Comparative example 3 201 483 42
The results in table 1 show that the steel of the invention obtains a single austenite structure and obtains higher high-temperature heat strength by regulating the Cr equivalent and the Ni equivalent, and the steel of example 1 has a dual-phase structure and reduced high-temperature strength because the Ni equivalent cannot meet the requirements. From the results of examples 2, 3 and 5, it can be seen that the strength at 550 ℃ is improved by adding Mo required for the inventive steel, and the strength is increased as the Mo content is increased. The steel of comparative example 1, although containing a high Mo content, did not contain C, Nb and Cu, and did not reach the strength level of the steel of the present invention.
As shown in FIG. 1, the microstructure of the steel of example 1 is a dual phase structure.
As shown in FIG. 2, the microstructure of the steel of example 2 is a single austenite structure.
The above examples and comparative examples show the endurance fracture times at 550 ℃ under a stress of 260MPa in Table 2.
TABLE 2
Figure BDA0003532678540000081
From the results of examples 2, 3 and 5, the invention steel has the advantages that after the required Mo is added, the durable fracture time at 260MPa and 550 ℃ exceeds 1000 hours, and the fracture time is longer along with the increase of the Mo content; however, if repeated hot forging or cold deformation is not used, or if the Nb content is too low, the permanent fracture time cannot exceed 1000 hours (see example 5 and comparative examples 2 and 3). Therefore, the steel of the invention needs component design and tissue regulation to obtain the best performance.
The above examples and comparative examples have an initial stress of 120MPa at 550 ℃ and residual stress values after 400 hours holding are shown in Table 3.
TABLE 3
Example 2 Example 5 Example 6 Comparative example 1 Comparative example 2 Comparative example 3
Residual stress/MPa 71 78 55 15 46 42
The results in Table 3 show that the residual stress after adding Nb, C and Cu to the inventive steel is significantly higher than that of the comparative example 1 steel in which Nb, C and Cu were not added as required, and the inventive steel exhibits excellent stress relaxation resistance. However, the best resistance to stress relaxation is not achieved with low Nb (example 6) or without tissue conditioning (comparative examples 2 and 3).
As shown in FIG. 3, the initial stress of the steels of example 5 and comparative example 1 was 120MPa at 550 deg.C, and it can be seen from the graph of residual stress versus time that the stress of example 5 decreased with time to a much lower degree than that of the steel of comparative example 1, i.e., example 5 had a higher residual stress and exhibited a more excellent stress relaxation resistance than comparative example 1.
The above examples and comparative examples show the values of the oxide film thickness after etching in a liquid lead bismuth alloy (45% Pb-Bi) at 550 ℃ for 3000 hours at a saturated oxygen concentration as shown in Table 4.
TABLE 4
Figure BDA0003532678540000082
As shown in fig. 4, 5 and 6, as can be seen from the morphology of the oxide film after the steels of comparative example 1, example 4 and example 5 are corroded in the liquid lead bismuth alloy (45% Pb-Bi) with the saturated oxygen concentration and the temperature of 550 ℃ for 3000 hours, the thickness of the oxide film of comparative example 1 is the thickest, the corrosion resistance of the liquid lead bismuth alloy is poor, the thicknesses of the oxide films of example 4 and example 5 are gradually reduced, and the lead bismuth corrosion resistance of example 5 is the best.
The results in Table 4 show that the lead-bismuth corrosion resistance of the inventive steel is obviously improved after Si is added (comparative example 1 does not add Si raw material as required); besides being related to Si element alloying, the lead bismuth corrosion resistance of the steel is found to play an important role in improving the liquid lead bismuth corrosion resistance of Mo in the steel for the first time. The addition of Mo has improved resistance to corrosion by liquid lead bismuth metal (examples 2 and 4), which makes the steel of example 5 have the thinnest oxide layer, although the Si content is not the highest, exhibiting the best resistance to lead bismuth corrosion.
The above embodiments are only for illustrating the technical idea and features of the present invention, and the purpose of the present invention is to enable those skilled in the art to understand the content of the present invention and implement the present invention, and not to limit the protection scope of the present invention by this means. All equivalent changes and modifications made according to the spirit of the present invention should be covered within the protection scope of the present invention.

Claims (10)

1. The liquid lead-bismuth corrosion resistant austenitic stainless steel for the high-temperature fastener is characterized by comprising the following chemical components in percentage by weight:
c: 0.06-0.12%; si: 2.0-3.0%; mn: 0 to 1.0 percent; s: 0 to 0.005%; p: 0 to 0.01 percent; cr: 13.0-17.0%; ni: 8.0-15.0%; cu: 0 to 1.0 percent; mo: 0.5-2.0%; nb: 8 is multiplied by 100C to 1.0 percent; o: 0 to 0.003%; n: 0 to 0.03 percent; the balance being Fe.
2. The liquid lead bismuth corrosion resistant austenitic stainless steel for high temperature fasteners according to claim 1, wherein the chromium equivalent is calculated according to formula (1):
cr equivalent weight 100 × (Cr + Mo +1.5Si +0.5Nb) (1)
The nickel equivalent is calculated according to formula (2):
ni equivalent of 100 × (Ni +30 × C +0.5 × Mn +0.5 × Cu) (2)
The Cr equivalent and the Ni equivalent satisfy: cr equivalent < 20; ni equivalent > 14.
3. The liquid lead bismuth corrosion resistant austenitic stainless steel for high temperature fasteners of claim 1, wherein the structure of the stainless steel is single austenite.
4. A preparation method of the liquid lead bismuth corrosion resistant austenitic stainless steel for the high-temperature fastener according to any one of claims 1 to 3 is characterized by comprising the following steps:
(1) mixing raw materials according to the requirements of chemical components, and obtaining a steel ingot through vacuum induction smelting and vacuum consumable smelting;
(2) homogenizing a steel ingot, wherein the homogenizing process comprises the following steps: the charging temperature of the steel ingot is less than 700 ℃, and the temperature is raised to 1100-1200 ℃ along with the charging temperature and is kept for more than 4 hours;
(3) forging the homogenized steel ingot at the initial forging temperature of 1080-1180 ℃ and the final forging temperature of 850-950 ℃, repeatedly forging the steel ingot at a large reduction in the longitudinal-transverse-longitudinal directions for not less than 3 times, wherein the single deformation is more than 10%, the total forging ratio is more than 20, and air-cooling the steel ingot to room temperature after forging;
(4) cold deformation with deformation of more than 30% is carried out after forging;
(5) and carrying out heat treatment after cold deformation.
5. The method for preparing the liquid lead bismuth corrosion resistant austenitic stainless steel for high temperature fasteners according to claim 4, wherein, preferably, in the step (3), the single deformation amount is more than 12%, and the total forging ratio is more than 25; in step (4), the deformation is > 35%.
6. The method for preparing the liquid lead bismuth corrosion resistant austenitic stainless steel for the high temperature fastener according to claim 4, wherein the heat treatment process in the step (5) is:
(1) preserving the heat for 0.5 to 2 hours at the temperature of 1000 to 1150 ℃, and cooling the mixture to room temperature in air;
(2) keeping the temperature at 800-900 ℃ for 2-4 hours, and cooling to room temperature in air.
7. The method for preparing the liquid lead-bismuth corrosion resistant austenitic stainless steel for the high temperature fastener according to claim 4, wherein the yield strength of the stainless steel at 550 ℃ is more than or equal to 200MPa, the tensile strength is more than or equal to 480MPa, and the elongation is more than or equal to 40.0%.
8. The method for producing a liquid lead bismuth corrosion resistant austenitic stainless steel for high temperature fasteners as claimed in claim 4, wherein the stainless steel has an oxide film thickness of not more than 30 μm after being corroded in a liquid lead bismuth alloy (45% Pb-Bi) at 550 ℃ for 3000 hours at a saturated oxygen concentration, and has excellent liquid lead bismuth corrosion resistance.
9. The method of making a liquid lead bismuth corrosion resistant austenitic stainless steel for high temperature fasteners as claimed in claim 4, wherein the stainless steel has a permanent rupture time of greater than 1000 hours at 550 ℃ under 260MPa stress.
10. The method of making a liquid lead bismuth corrosion resistant austenitic stainless steel for high temperature fasteners as claimed in claim 4, wherein the stainless steel has an initial stress of 120MPa at 550 ℃ and a residual stress of more than 70MPa after 400 hours holding.
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